Common causes of pump failure
How bearings, couplings and seals fail – and how to fix them.
Excessive vibration, loud screeching, higher-than-normal heat emissions, off-curve pressure readings, and leaking process fluids are all signs that pump failure is imminent. To avoid further damage to the pump and downstream equipment, as well as costly downtime repairs, it is essential to find the root cause of the pump problem and quickly restore it to peak performance.
Bearings, couplings, and seals are the most common pump components that fail. Experience has shown that ignoring these items will not only increase maintenance costs, but also increase operational costs in terms of resources and downtime. Here’s how to identify those component failures, discover the root causes, and prevent them from happening again.
1. Bearings: How they fail
Many red flags may indicate a bearing blowout problem: increased vibration coefficient, louder than normal operation, and increased temperature around the bearing housing are all common warnings. Pay attention to these little signs. Do not ignore even minor noises. When a loud screeching sound is heard, the bearings are usually shot and the pump is about to lock up.
While bearings are normally lubricated at the factory, they require a separate lubrication schedule depending on the application and the pump’s operating schedule. (An exception to this is sealed bearings.) Failure to properly lubricate the bearings can lead to overheating and premature failure.
An additional threat to proper bearing performance is lubricant contamination. Sand and other contaminants can damage the bearing surface and cause inconsistent performance and reduced bearing life. A less common threat to bearings is misalignment and vibration, which can cause scaling and damage to the bearing surface over time.
How to avoid failure
Set up a regular lubrication schedule and record the lubrication in a maintenance log. Avoid the common lubrication mistakes described below.
The number one lubrication mistake made in this area is adding grease while the pump is running. If the pump is lubricated during operation, the grease will not reach the rolling elements and the bearings will dry out. Always turn off the pump before adding grease to the bearings.
Never mix different types of grease. Different grease thickeners can be incompatible and cause starvation or hardening.
Do not over grease the bearings. Over-lubrication can be just as harmful as under-lubrication. Excessive lubrication can cause excessive heat and premature bearing failure. (Note that sealed bearings do not require additional grease; if grease is added to sealed type bearings, this can dislodge the seal.)
Check the lubricant periodically. If it appears contaminated, analyze it to determine what type of contaminants are entering the bearing housing. Once the type of contamination is identified, take corrective actions to prevent its recurrence.
Good system design and solid mounting should prevent excessive vibration that can lead to bearing spalling or pitting. Pumps must be securely mounted on a level base, and great care taken to support the inlet piping to avoid any strain on the pump. The pipe hanger installed on the discharge pipe must bear all the weight, not the pump or body. See Figure 1 for proper installation and support. Regularly monitor equipment for changes in vibration and temperature to identify changing conditions.
2. Pairs: How they fail
When the pump shaft and motor shaft are out of alignment, the couplings fail. They may be misaligned from the start due to improper installation, or become so over time due to system vibration. One way to visually identify a faulty coupling is to look for black debris under the pump coupling area. This is from the coupling insert that is placed between the coupling flanges. In an unbalanced coupling, the surfaces of the flanges rubbing against each other will, over time, crush the insert and create a pile of chips.
Another key indicator of faulty couplings is vibration. Any vibration higher than that normally observed during regular pump operation should be investigated.
How to avoid failure
Coupling alignment should be checked whenever a problem is suspected – due to chips or the aforementioned vibrations – as well as part of a routine maintenance schedule. Whenever you repair a pump, proper alignment should be verified both before start-up and after the pump has reached operating temperature (called a “hot alignment”).
Parallel coupling alignment should be checked first by placing a straight line on the two coupling flanges and measuring the maximum offset at various points around the coupling. If the maximum offset exceeds that specified by the coupling manufacturer, the coupling must be realigned.
After the parallel alignment is satisfied, the angular alignment of the coupling should be checked with a micrometer or caliper. Measure from the outside of one flange to the outside of the other flange around the circumference of the coupling. If the difference between the maximum and minimum exceeds the tolerance specified by the coupling manufacturer, the coupling must be realigned. If correction is necessary, parallel alignment should be rechecked. See Figure 2 for proper coupling alignment techniques.
3. Mechanical seals: how they fail
Mechanical seal failures are usually very easy to detect – a slow drip or sometimes a steady stream of process fluid originating from the seal gland is a dead defect. Seals account for most of the failures of rotating equipment during the life of the equipment, but are rarely the main cause of failure.
Choosing the wrong sealing material for the process is a common mistake. Improper selection can result in swelling of seal O-rings or cracking or corrosion of the seal surface. While most people know that they choose a seal based on the process fluid—for example, a stronger seal is needed for a corrosive fluid—many don’t consider the operating conditions. A fluid such as water is usually considered inert. However, add too much temperature
and flashing and damage to the sealing surface may occur.
Another temperature consideration that can affect a seal, even if the seal material is properly selected, is related to operating conditions. Taking a stopped pump from ambient temperature to immediate operation in very high temperature or supercooled fluid can thermally shock the seal and cause cracking.
Pumps that move more viscous fluids such as paint can see build-up along the face and edges of the seal. Over time, this can gum up the seal and cause failure. See Figure 3 for an example of a seal that failed due to paint build-up.
Beyond these specific issues of seal selection and application, the primary cause of seal failure is dry seal running, which can cause thermal shock or burnout of seal elastomers. Any number of systemic problems can lead to dry running. Low fluid level from the source, blockage on the suction side of the pump, or operating with a closed drain (dead target) can all cause temperature rise and seal failure.
How to avoid failure
The first step in preventing mechanical seal failure is to select the appropriate seal for the process fluid and operating conditions. Review the application in detail with the application sales engineer instead of default selection based on fluid type.
If extreme temperatures are involved, consider changes to the setup procedure to avoid thermal shock to the seal. Gradually introduce the process fluid into the pump so that all components reach full temperature at a slower rate. Adding external heating or cooling elements is another possible solution.
For applications prone to process buildup on the seal, an external seal wash system can be added to prevent those particles from sticking to the seal. If there is an external flush system on a pump, it should be kept in good working order and checked for leaks and damage.
Once the proper sealing material has been selected for the application, the focus should shift to the suction side of the equipment to avoid dry running. If the fluid in the supply is regularly low, a level control switch should be installed to prevent low supply level. If drain current or pressure drop is observed, this is an indication that the inlet should be checked for blockage. Except for short periods to test the pump’s performance, the pump should never be run at closed flow – symptoms and care procedures can prevent this from happening.
One solution to troubleshooting sealing problems is retrofitting the process with a floodless pump. These enclosed vertical column designs reduce leakage through the throttle bushing, collecting any accidental leakage in the pump column and discharging it to the suction or feed tank. Vertical submersible pumps are suitable for a wide range of challenging applications – even those with high solids or high temperature fluids. Retrofitting a horizontal pump with a vertical enclosed column pump is usually accomplished with minimal piping changes and the addition of supports for external mounting configurations.
Conventional wisdom holds that those who do not learn from history are doomed to repeat it – this applies to pump maintenance and repair. Documenting repairs can help identify future problem pumps and help diagnose future pump failures. This important final step is often overlooked to get a pump up and running again.
There are several key elements to consider. The more details that are documented, the easier it will be to accurately identify those outliers. For starters, maintenance notes should include any unusual operating conditions around the timing of the repair problem (eg, a holiday shutdown period, unusually high temperatures, etc.). Note the location of the pump in the process and the reason the pump was identified for repair (stop, performance, leakage, noise, amps, etc.).
Repair notes should also include diagnosis and actions taken to repair the pump. Once identified as a problem pump, it can be flagged for more detailed inspection of installation, piping, operation and repair procedures. This more detailed inspection should lead to the identification of the root cause of the failures.
Using historical data to eliminate recurring problems increases pump life and prevents process failures.